The present invention pertains to a stratified scavenged, small displacement two-cycle internal combustion engine for powering portable power tools and equipment used in forestry, lawn and garden, construction and transportation.
The two-cycle engines has been for a long time the preferred choice for powering portable tools and small high performance vehicles due to high specific power, mechanical simplicity, low maintenance and relatively low cost. The recent introduction of tougher exhaust emission regulations has forced manufacturers to develop systems that ensure complete combustion and minimization of scavenging losses.
Since the invention of the two-cycle engine in the late 1800""s, the scavenging losses has been recognized as a problem, since then many solutions have been suggested in order to improve the fuel efficiency. One of these early solutions was attributed to the inventor Sir Dugald Clerk who suggested the idea of an xe2x80x9cair headxe2x80x9d as buffer of clean air followed by the scavenging gases to minimize the losses of combustible mixture. Following his idea, in 1911, Stephenson was granted the patent U.S. Pat. No. 1,012,288 using the xe2x80x9cair headxe2x80x9d principle to improve fuel efficiency in a two-cycle engine. In 1922, Lanchester and Pearsall presented an xe2x80x9cair headxe2x80x9d idea similar to recent developments where the air is admitted through the transfer ports. xe2x80x9cAir heads enginesxe2x80x9d, has been also called stratified scavenged engines due to the separation of the scavenging flow in two different charges: one substantially lean followed by a slightly rich fuel/air mixture.
The introductions of demanding exhaust emissions regulations all over the world has triggered the interest in the xe2x80x9cair headxe2x80x9d concept. Modern versions have shown substantial gains in fuel efficiency and emission reductions. It is important to recognize the work of scholars of the Queen""s University in Belfast who have provided significant amount of research in this area and have published numerous technical papers.
Following two classical solutions applied to modern versions of stratified scavenging engines are illustrated by FIG. 1, showing two typical prior art stratified scavenged engines. FIG. 1a displays a simplified view of an engine as described by the U.S. Pat. No. 6,112,708 patent by Sawada. This engine works under the traditional principle of filling up with fresh air the volume of the transfer passages 24 located in both sides of the cylinder block 12, and discharging them into the cylinder chamber 14 during the scavenging cycle. The air is admitted into the transfer passages 24 through one way air valves 28 located at the top portion of the transfer passages 24 and at the end of the air intake passage 36. The airflow being controlled by an airflow control valve 38 during the induction period of the engine. At the beginning of the scavenging period, the volume of air previously admitted into the transfer passages 24 is displaced by the air/fuel mixture pressurized into the crankcase chamber 11, creating an xe2x80x9cair headxe2x80x9d to displace the remaining combustion gases into the cylinder chamber 14. As previously mentioned, this method was suggested by Sir Dugald Clerk in 1989, to reduce hydrocarbons losses. This style of stratified scavenged engine is costly to manufacture due to the special closed transfer ports cast into the cylinder, the required hardware required for its construction and special materials used for parts under high temperature levels such as the air intake valves.
FIG. 1b shows another style of prior art stratified scavenged engine as described in the U.S. Pat. No. 4,253,433 patent by Gordon Blair. This engine utilizes secondary transfer ports 22 to discharge a lean air/fuel mixture into the cylinder chamber 14 prior to the discharge of the main transfer ports with a slightly rich air/fuel mixture. The enleanment of the air/fuel mixture discharged through the secondary transfer ports 22 is accomplished by centrifugation, which separate the heavy fuel particles from the air/fuel mixture. This separation is accomplished by forcing a portion of the air/fuel mixture pressurized into the crankcase chamber 11 through a curved long transfer passage 24. The timing between the secondary and main transfer ports is controlled by the piston motion. As in the previous example, the lean mixture displaces the residual combustion gases into the cylinder followed by a stratified rich mixture to complete the scavenging process and fill the cylinder chamber 14 with combustible mixture. This style of engine is also costly to manufacture due to the closed transfer passages used on the cylinder and the two parts crankcase required to form the extension of the transfer passage into the crankcase. In addition to the manufacturing cost, catalysts must be added to reach the very low levels of hydrocarbon emissions required by the new government regulations.
For more than a century many other versions of stratified scavenged engines utilizing the xe2x80x9cair headxe2x80x9d principle, has been proposed. All these prior art engines utilize complex structures that depart from the simplicity of the traditional two-cycle engines. The present invention teaches a new xe2x80x9cair headxe2x80x9d design method using concepts aimed to the minimization of cost, weight and package size.
It would be obvious to the person skilled in the art, that the prior art of stratified scavenging in two-cycle engines, often has a complex construction not desirable for hand held portable tools where compactness, weight, simplicity, cost and low emissions are the dominating factors.
The object of the present invention is to provide a lightweight, compact and economical stratified scavenged two-cycle engine that offers substantial advantages over similar art such as lower weight, cost and package volume. Its simplicity and purposeful construction allows a low manufacturing cost as required in hand held gas powered tools as those used in construction, forestry, lawn and garden applications and in light transportation systems.
The two-cycle, stratified scavenged internal combustion engine object of the present invention comprises all the basic elements of typical crankcase scavenged two-cycle engine with very few modifications. The cylinder bore contains in addition to the exhaust ports, transfer ports and a combustion chamber, a fresh air supply port which is added to the cylinder wall as a mean of clean air supply. The piston is modified to allow cavities for induction, holding and ejection of the fresh air volume followed by combustible mixture.
The piston passages have two fundamental functions: to create a labyrinthine path for air to be induced through, and as a flow path for the scavenging gases from the crankcase. Air is induced into the piston passages in the proximity of the piston top dead center position (TDC), by the crankcase vacuum which pulls air through the labyrinthine passages within the piston. When the piston is in the proximity of the Bottom Dead Center position (BDC), the same point of entry of the air into the piston passages and the lateral piston chambers become in communication with the crankcase, allowing the pressurized air/fuel mixture into the crankcase to sweep the air trapped into the piston cavities or passages. By virtue of this process, the air held within the piston cavities is displaced and pushed into the cylinder transfer ports (also called scavenging ports) during the scavenging period. As a result, a blanket of air or an xe2x80x9cair headxe2x80x9d, is used to displace the exhaust gases out of the cylinder, minimizing the escape of fuel/air mixture, therefore, reducing HC emissions.
The engine object of the present invention offers several obvious advantages over the prior art xe2x80x9cair headsxe2x80x9d designs: 1. There are not valves exposed to high temperature levels as those encountered at the top of the transfer passages of some prior art engines. 2. The circulation of scavenging gases through the piston walls will provide added lubrication and cooling capacity prolonging engine life. 3. Added lubrication and cooling capacity allows the engine to run at leaner settings, therefore minimizing the hydrocarbon losses. 4. The construction of air passages internal to the engine offers space savings and simplicity which will allow for lower cost and easy adaptability to existing shrouds. 5. The mechanically interrupted flow of air and scavenging gases, avoid tuning losses as found in typical prior art engines.
Due to the use of fresh air to scavenge the exhaust gasses, some of the fresh air escaping through the exhaust port as scavenging losses, mixes with the exhaust gases, which contains high levels of carbon monoxide. As a result of this chemical reaction, the excess air into the exhaust gases stream oxidizes significant amounts of carbon monoxide. The carbon monoxide is then transformed into carbon dioxide, which is a harmless gas.
The preferred embodiments of this invention have several inventive aspects, which jointly contribute to the main functional object of the invention: to reduce exhaust emissions and improving the fuel efficiency while reducing cost and weight of the engine. The invention will be better understood and further objects and advantages thereof will become more apparent from the ensuing detailed description of preferred embodiments taken in conjunction with the drawings.